Cryptography

1
Cryptography

• The encryption model (for a symmetric-key cipher).

2
Transposition Ciphers

• A transposition cipher.

3
One-Time Pads
The use of a one-time pad for encryption and the
possibility of getting any possible plaintext
from the ciphertext by the use of some other pad.
4
RSA Public-Key Algorithm
1. Choose two large primes, p and q (typically
1024 bits) 2. Compute n p x q and z (p-1) x
(q-1) 3. Choose a number realtively prime to z
and call it d. 4. Find e such that e x d 1 mod
z To encrypt P C Pe (mod n) To decrypt C P
Cd (mod n) (e, n) is the public key and (d,
n) is the private key. You freely distribute
your public key.
5
Using Public-Key Algorithms
Encrypted message
M
EBpub
B
A
Bpub
Bpriv
Apriv
Apub
Signed message
M
hash(M)
EApriv
Certificates
Apub
E(signing authority)